Wellesley residents have been told to boil their tap water and will likely need to continue to do so through the weekend after E. Coli bacteria was discovered Thursday morning.

“It will be at least the weekend before we’re able to lift the ban, in my opinion,” Wellesley’s Executive Director Hans Larsen said Thursday afternoon.

“It’s going to take us some time to go through the various diagnostic procedures and continued sampling,” said Larsen. “We hope to resolve the source of the contamination and prove that the water supply is safe... I think it’s likely to take a couple of days here to resolve.”

Larsen, who stressed that he’s not an expert on water safety, said that state and town officials would need to run several rounds of testing that shows the water is safe before lifting the ban. The directors of the Health Department, Department of Public Works and the Water Superintendent were all unavailable for comment Thursday.

According to the announcement from the town, residents should boil their water for at least one minute before drinking. People with increased risks—such as elderly individuals and those with weakened immune systems—should consult with their doctors, the release said.

E. Coli is bacteria that can cause diarrhea, cramps and other ailments, the town’s announcement said. For more information on precautions to take, click here.

The contamination is believed to be isolated to one tank near the town’s reservoir which contains millions of gallons of water, but the town’s water system

Some residents took to social media Thursday afternoon to lament what they called a slow effort by the town to notify the public about the need to boil water. Town officials first became aware of a problem around 9 a.m. Thursday, Aug. 21, but formal word was not released to the media until after 2 p.m.

Residents said they had not received a reverse 911 call from police until close to 4 p.m.

According to Larsen, the gap arose out of a need to secure formal notification from the state’s Department of Environmental Protection, which first posted the order on its own website. The town did notify residents well within a required 24-hour window of the discovery of the E. Coli, Larsen said.

“You can’t just put a notice out,” Larsen said. “You’ve got to have places for people to call…to go and get additional information. All of that has to be set up and structured before you start putting out announcements.”

Page 2 of 2 - “We wanted to have something to point residents to as an additional source of information,” Larsen said. “There is some amount of boilerplate material that’s available that needs to be made situation-specific.”

With schools due to open next week in Wellesley, Superintendent David Lussier said that the water ban would not affect the start of school, even if it were to continue beyond the weekend.

"We've already prepared to have large quantities of bottled water at schools on Monday when all staff report back and would continue to do so as needed," Lussier told the Townsman Friday. "Our bathrooms are also stocked with hand sanitizer for hand washing."

"We're certainly hoping for a resolution this weekend," he said, "but we're ready for next week regardless."